1. Field of the Invention
The present invention relates to a pneumatic radial tire including at least one ply of carcass layer comprising a number of cords disposed at right angles with respect to the circumferential direction of the tire and anchored at a pair of beads, and more particularly to a highly durable pneumatic radial tire, especially a highly durable radial tire adapted for use on a large and heavy vehicle, the durability of which is greatly enhanced by an improvement in the treatment of carcass cords.
2. Prior Art
A carcass layer which is one of the important constituents for a pneumatic radial tire is composed of carcass cords and a covering rubber coated thereon. Polyester cords are often used for carcass materials.
The strength of polyester cords, however, is 5 to 7 g/d and of comparatively low strength. Accordingly, a pneumatic tire used for a large heavy vehicle has to include at least two or more plies of carcass layer. As a result, a carcass layer becomes thicker and causes a higher heat build-up which results in deterioration of the durability of a pneumatic tire.
On the other hand, steel cords having comparatively high strength, one ply of carcass layer may be applicable. However, a reduction of tire weight is impaired. Besides, the steel cords being so corrosive, reduction of strength is apt to be caused.
Recently proposals have been made to use aromatic polyamide fiber cords of much higher strength than polyester cords. One ply of a carcass layer will do so that both a reduction of tire weight and an improvement of durability are attained together.
Aromatic polyamide fiber cords, however, are rather inferior in adherence to a covering rubber as compared with polyester cords and steel cords for the reason of the molecular structure of aromatic polyamide. Then there is an anxiety that a separation trouble occurs between carcass cords and a covering rubber.
Moreover, a turn-up end of a carcass layer turned up around a bead exposes a cut face of cords which is free from adhesives so that a turn-up separation from a covering rubber or a surrounding rubber occurs at an early stage and the durability of tire is much impaired.